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Wind shear is a natural weather phenomenon that occurs in an earth's atmosphere. It is a kind of turbulence. That affects the flight of the aircraft. It is a sustained change in the wind resulting in the changes of the lift in the aircraft. People who have flown might have experienced sudden change in the flight altitude this is because of turbulence caused by wind shear. In commercial aircraft pilot has to respond to this phenomenon quick to ensure the safety of the passenger and the aircraft itself. Many accidents have been recorded in the past due to the wind shear. So it is very important to study this phenomenon in aviation industry.
This report explains about the wind shear and it's types. It looks at why study of wind shear and weather condition is important to aviation industry. This report also explains about the technologies out there to detect the wind shear and how it could be prevented. It also explains the reaction of the pilot on this event and what are the instructions and training given to deal with this situation.
Wind shear is magnitude of a wind change with height from point to point. This basically means change in wind speed or direction as you go higher and higher. It also refers to change in head wind or tail wind. Significant changes of head wind could result in significant changes in the lift of the aircraft and which could lead to the changes in the intended path of the flight. As shown in the figure 1.1.
Metrological studies have confirmed the existence of burst phenomena. These are basically small-scale downdrafts. Which when reaches the ground spreads vertically and horizontally. Which is extremely dangerous to aircraft. Wind shear causes extreme risk to the aircraft within 1000 ft. Altitude particularly in approach to the landing and take off. During take off aircraft encounters head wind after that downdraft and tail wind. Which could lead to the loss in the desired lift by the aircraft. Under these conditions pilot experience difficulty in controlling the aircraft so they will have to correct the disturbance perfectly.
A survey conducted by international civil aviation organization (ICAO) in 2005 has shown there have been 28 fetal aircraft accidents between 1964-1983 below 1500ft. resulting in 500 fatalities and 200 injuries. Consequences of wind shear accidents could be fetal so it is considered as on of the biggest hazards in aviation industry. and also most looked and researched topic in the industry. Many research projects have been carried out to minimize the accidents since then. And seems to have been working as there is significant reduction in accidents.
2 Types of wind shear.
There are various types of wind shear that occurs in earth atmosphere some of them, which have major impact on aircraft, are explained below.
2.1 Terrain induced winds hear.
When high-speed wind stream is obstructed by the large buildings or mountains this phenomenon occurs changing the direction of the wind and the reducing the speed of the wind. But some of the wind escapes from the gap in between and with the low speed wind in the middle of this disturbed high-speed wind on the other side wind shear if formed.
And when aircraft passes through this alternating low-speed wind stream to high-speed wind stream a significant amount of headwind gained is experienced which could lead to the undesired generate of lift on the aircraft. And when aircraft flies from high-speed wind stream to low-speed wind stream a significant loss of headwind is experienced resulting in a sinking motion. As shown in fig 2.1.1.
2.2 Sea breeze wind shear
In a day time near coastal area ocean get too much sun heat and water starts evaporating which forms the sea breeze and when this breeze encounter the warm breeze from the land then wind shear is formed. Airport near the coastline experiences these shears. When aircraft approach to landing it experience headwind gain, which generates lift, which could lead to the overshoot on landing. As shown in fig 2.2.1.
2.3 Microburst wind shear
Microburst wind shear is the most extreme form of thunderstorm and most dangerous to aviation industry.
It is the downstream flow of extreme cool wind in a very high speed. And causes the intense horizontal outflow of the wind in a very high speed of about 75m/s covering a large area. In an extremely dry condition winds may exceed hurricane force. And may approach the speed of medium tornado. In a wet condition microburst may be embedded in a heavy rain. But it's onset may be so sudden as to catch pilots unaware.
When a aircraft passes through the microburst it experiences a headwind and lift followed by the downdraft from above and after that tailwind and sink. Microburst could be asymmetry strong wind in one side and weak in the other. In some cases downdraft from microburst could hit the ground at an angle rather than vertically. When aircraft passes through this than it may experience wind shear differently.
More than 500 fatalities have died since 1970 in USA only. So if weather looks suspicious than most of the pilots wouldn't take off or land. Example of microburst is shown in fig 2.3.1.
2.4 Low level jet wind shear.
Low-level wind shear is a narrow band of strong shear that occurs in the lower atmosphere. Most of the aircraft experiences this shear during landing and takeoff. During ascending aircraft experiences headwind and lift. As aircraft leaves the shear headwind and lift drops. It is similar for the descending aircraft as well. However descending aircraft descend at a lower angle than that of ascending one (i.e. 3 degree glide path) the changes in headwind experience is also lower. As shown in fig 2.4.1.
It is very difficult to detect any kind of wind shear. Many effort and development have been going on to make sky safer. There are different types of detection systems installed in the airports all around the world.
Some of the systems are explained in this report.
3.1 Terminal Doppler Weather Radar(TDWR).
Microburst is an intense downdraft that is sometimes generated by thunderstorm. If an aircraft encounters this shear when flying in low altitude it may lose altitude rapidly and not be able to recover. To avoid this sort of disaster TDWR has been developed throughout the years. Which is now deployed all over the world.
This system was developed by Lincoln laboratory and manufactured by Raytheon.
TWDR detects any raindrop, ice, snow, and hail also known as hydrometeors in the atmosphere. By emitting pulsed microwave of 5cm wavelength. It analyses the intensity and the frequency shift of reflected wave and calculate the concentration and movement of hydrometeors.
Besides it's primary use of detecting microburst its data is also used by weather integration systems. Such as Integrated Terminal Weather System (ITWS) and Corridor Integrated Weather System (CIWS).
TWDR excellent narrow beam and ground clutter suppression provide high quality data on boundary layer dynamics. Which is a key to improving convective initiation forecasting and small tornadoes. Its increased computing capability has also enabled the use of more advanced signal processing techniques to tackle difficult data quality issue such as range velocity ambiguity and unwanted signal clutters formed by moving objects such as birds.
For the better use and data quality of TWDR. Innovative ideas such as adaptive waveform transmission and processing. Typical TWDR is shown in fig 3.1.1.
3.2TWDR technical specifications
Since TWDR is very effective in detecting wind shear in rainy condition. It is not as effective in dry conditions. It is because of absence of hydrometeors in the atmosphere.
So to prevent any disaster another technology is introduced called LIDAR (Light Detection and Ranging).
3.3 Light Detection and Ranging (LIDAR).
LIDAR is high accuracy, high definition, and high-resolution pulsed Doppler system. Which is used to detect wind shear in dry conditions. It works similar to TWDR. It emits laser pulses of wavelength 2μm in moving micro particles also known as aerosols to detect wind speed and direction. It works by scanning at several different elevation angles to monitor the wind conditions of about 3 nautical miles. It works by scanning the take off and landing path of the runway. It works parallel with highly sophisticated computer systems to give accurate data. LIDAR is also used in many weather related conditions. And has proved effective now and then it is considered as one of the best supplements to TWDR system.
Example of LIDAR scanning is shown in fig 3.3.1.
Fig 3.3.1. LIDAR detection of a sea-breeze.
3.4 TWDR and LIDAR are just two examples of wind shear detection system that has been installed on the ground. There are many other systems could be found in airports and in development for the secure and safe flight of an aircraft. These two systems are ground based but there are other technologies, which can detect wind shear in air. Some of the are explained below.
3.4.1 On Board wind shear warning system.
Pilots need between 10-40 seconds of warning to avoid wind shear. Shorter that 10 seconds could be fetal and longer than 40 seconds is too early to detect anything as atmospheric conditions could change.
NASA has test flight-tested three systems and found it effective to avoid wind shear. Overview is shown in fig 220.127.116.11.
Fig 18.104.22.168. Overview of onboard wind shear warning system.
3.4.2 Microwave Radar
It sends microwave radar ahead of the aircraft to seek for the raindrops and other moister particles. Returning signal represents the movement of those raindrop and moisture particles, which is then translated into wind speed. This system works better than other system in rainy or moist conditions. But it's not that reliable in dry weather. Because this system points the ground as a plane surface, it picks up any interference or clutter. But more research is going on to eliminate this problem. A example is shown in fig22.214.171.124
fig 126.96.36.199. Microwave radar sensor in the nose of Boeing 737
3.4.3 Doppler LIDAR
It is a laser system that fires laser into the aerosols in the atmosphere instead of raindrops. This system can avoid picking up the clutters or interference. Only drawback to this system is that it doesn't work very well in heavy rain. An example is shown in fig 188.8.131.52
fig 184.108.40.206 A LIDAR sensor is mounted in the belly of 737.
This system uses infrared signals to detect the thermal signatures of carbon dioxide ahead of the aircraft. To look for the cool columns of air which are microburst. This system is less complex that other systems and due to its complexity it is inexpensive. A infrared sensor is shown in fig
Fig 220.127.116.11 infrared sensor mounted on the side of 737.
3.5 Wind shear and Turbulence Warning System
TO assists pilots for safe take off and landing during wind shear airports have to be fitted with latest wind shear detecting systems such as LIDAR and TDWR.
During Wind shear or microburst these system collects and analyze weather data and report to the air traffic controllers who are the main frontline communication to the aircraft. And then they direct pilots in accordance to avoid the turbulence.
Pilots are alerted generally in tow ways
Wind shear alerts are directly passed to the aircraft via ATC (Air Traffic Controller) within 3 nautical miles of the runway.
Automatic Terminal Information Service (ATIS)-Wind shear warning issued by aviation forecaster is directly broadcast to aircraft on voice-ATIS and Digital ATIS.
Wind shear and turbulence warning system is shown in the example shown in fig 3.5.1.
Fig 3.5.1 wind shear warning system.
Wind shear is a natural phenomenon that has been occurring since millions of years and will continue to occur as long as the existence of the human planet. It could be predicted and avoided using different technological systems like TWDR and LIDAR. But most of all pilots should be given appropriate tanning to deal with wind shear situation. There has been lots of research going on with the local aviation company and the government of different countries to make sky safer. We cannot defy nature but at least with the help from cutting edge technology and human brain we can avoid it and save lots of life.